Most commonly the betrayer is referred to as an adulterer, although cheater works as a more colloquial term.

What is the person your cheating with called?

A man who cheats on his wife is an adulterer. A woman who cheats on her husband is an adulteress. An adulterer commits adultery with his mistress, or lover, or paramour or girlfriend.

What is the meaning of cheating in relationship?

[Cheating is] to be emotionally or physically unfaithful. There are degrees of cheating from sexual betrayal of a partner or spouse to affairs of the heart in which a member of a relationship has a secret, emotionally meaningful relationship outside his or her primary one. — Jeffrey Rubin, Ph.D.